1. Field of the Invention
The present invention relates to an electromechanical interferometric load-pull tuner system, and more particularly to such a system to be used in load-pull setup for the measurement, characterization and testing of RF (Radio-Frequency) or microwave devices. RF/Microwave tuners are electronic devices or mechanical devices which modify in a predictable way the complex impedance seen by the device under test (hereinafter referred to as “DUT”) at a given frequency of operation. The interferometric load-pull has the capability of generating impedance to the microwave devices close to the conjugate complex of the DUT's internal impedance. This technique of subjecting DUT to variable load impedance or variable source impedance with corresponding load tuner and source tuner, commonly referred to as “load pull”, is used to test transistors for amplifier, oscillator or frequency multiplier applications.
2. Description of Prior Art
A manual tuner with a manually extractable parallelepipedic dielectric slug has been described by KESHISHIAN Richard in “VSWR Tuner” MACOM Application Note AN0004. The manual tuner is constructed as a 50 ohm impedance air line (1), which contains a unique dielectric slug (3) sliding on a central conductor (2) and uses different slugs with different impedances, i.e. built with different pair of dielectric slabs (4,5), to transform the 50 ohm impedance to a pre-determined reflection amplitude and phase. The slug with its two identical dielectric slabs (4,5), basically acts as a shunt capacitor in parallel with the 50 ohm line. The value of this slug capacitance determines the value of the VSWR of the tuner, while the position of the slug in the transmission line dictates the phase. The unique slug is moved along the tuner, as indicated by arrow (3′), to provide full 360° VSWR circle coverage on the Smith chart. To get this full Smith chart coverage, the tuner must be designed for 180° minimum phase length at the lowest required frequency of operation. The value of the VSWR that each air/dielectric slug will provide is related to the permittivity Er of the slug dielectric and the air gap being left between the dielectric and the outer conductor.
To overcome the very time consuming process of using manual tuners, the slugs can be driven by motors controlled by a computer. A first automatic double-slug tuner is discussed in the article “Automatic Load Contour Mapping for Microwave Power Transistors” by J. M. Cusack, S. M. Perlow and B. S. Perlman, in IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-22, No. 12, December 1974. The tuner consists of a uniform coaxial line with characteristic impedance Z0 of 50 Ohm (6). The line contains two identical moveable slugs (8,9) that slide, along the longitudinal axis of the coaxial line as indicated by arrows (8′,9′), between the inner conductor (7) and the outer conductor (6), said inner conductor being the central conductor (7) of the coaxial line. The sections of line containing the slugs have characteristic impedance, Zs, which is lower than Z0. The cylindrical slugs could be purely dielectric (8,9) or dielectric (10,13)/metal (11,14)/dielectric (12,15) and are surrounding the central conductor (7) therefore being not extractable from the coaxial line. As the two slugs can not be extracted from the coaxial line, it is inevitable to calibrate the tuner at high gamma values and it is well known that vectorial network analyzers required for measuring the S-Parameters during the calibration become less and less accurate when very high reflection factor loads are to be measured, therefore limiting the possible accuracy of this calibration method.
The automatic slide-screw tuner consists of a coaxial 50 ohm characteristic impedance slabline (16), round center conductor (16), two slabs as outer conductor ground return (16) and uses only one metallic slug (17), movable in horizontal (17′) and vertical (17″) direction into a transmission line. It has been first described by LANGE Julius in “Microwave Transistor Characterization Including S-Parameters”, Texas Instruments, Hewlett Packard Application Note 95. The search algorithm is extremely simple since the horizontal move of the slug corresponds to a phase variation of gamma, while the vertical move of the slug corresponds with an amplitude variation of gamma, making the impedance synthesis much easier compared to the previously described double-slug tuner. The main disadvantages of slide-screw tuner are power limitations and high gamma accuracy limitations. Power limitations are due to corona discharges that can take place between the tuning metallic slug (17) and the central conductor (18) at low impedance, i.e. high gamma. The greater the distance between the metallic slug and the central conductor, the more power can be transmitted, and conversely, the smaller the distance, the less power can be transmitted mainly because of corona discharge. Accuracy limitations also at low impedance are because of mechanical limitations to reproduce accurately a given slug position close to the central conductor.
One enhancement of slide-screw tuner is disclosed in C.A. Pat. No. 2,311,620 and U.S. Pat. No. 6,674,293 issued to Christos Tsironis on Jan. 6, 2004, which relates to a double slide-screw tuner that comprises a first (21) and a second (22) slugs in series in a transmission line (19), said first slug is adapted to act as a pre-matching section and said second slug is adapted to act as a tuning section. But as described in the patent, this technique being in reality two independent tuners connected in series with 4 axis of freedom, 2 horizontals (21′,22′) and 2 verticals (21″,22″), the combined calibration time may be too long for practical considerations. Consequently, the patent provides alternative methods for calibrating the tuner to cut down the calibration time. But these calibration methods are based on approximations and give limited accuracy at high gamma. Also this tuner requires a additional sub-micron vertical positioning system for the second slug compared with the central conductor (20) which increases considerably the mechanical construction difficulty and the manufacturing cost.
Accordingly, there is a need for an efficient, economical, simply constructed and improved impedance tuner. It is another primary object of the invention to provide an exact three-terms procedure to calculate the S-parameters of the tuner with slugs displacement.